1. Field of the Invention
This invention relates to a storage system and to a remote copy control method for a storage system.
2. Description of the Related Art
In the computer systems of recent years, the quantities of data used are increasing, and data is also updated with increasing frequency. Hence methods for backing up such data, or for promptly restoring a normal operating state after occurrence of a fault, are vital issues for storage technology.
As one solution to such problems, technology has been proposed in which, for example, a plurality of storage control devices are placed in both a primary site and a secondary site, mutually separated, each of the storage control devices at the primary site (primary storage control devices) and each of the storage control devices at the secondary site (secondary storage control devices) are connected by transfer paths, and data managed at the two sites is synchronized (Japanese Patent Laid-open No. 2004-13367). In this technology of the prior art, a so-called remote copy technology is used in which data updated by a primary storage control device is transferred and copied to a secondary storage control device, without passing through a host computer (hereafter “host”).
As technology to ensure consistency between primary storage control devices and secondary storage control devices, a method using the write time has also been proposed (European Patent No. 0671686). In this technology of the prior art, a primary storage control device which has received write data from a primary host reports completion of reception of write data to the host immediately after receiving the write data. The primary host, after receiving the report from the primary storage control device, reads a copy of the write data from the primary storage control device. The time at which the write request was issued from the primary host (the write time) is added to the read-out write data. The primary host transfers the read-out write data and write time to a secondary host. The secondary host references the write time added to each write data set, and writes the write data sets, in the order of the write time, to a secondary storage control device. By this means, consistency is maintained between data stored in the primary storage control device and data stored in the secondary storage control device.
In the above technology of the prior art, because the primary host and secondary host constantly perform data transfer and reflection processing (processing to write data to a volume in the order of the write time), the load on each host is considerable. It is necessary that each host always be running, and moreover it is necessary that software to execute the above-described data transfer processing and reflection processing always be running.
When remote copy paths are set between a plurality of primary storage control devices and a plurality of secondary storage control devices, and parallel processing of a plurality of remote copy tasks is performed simultaneously, at a certain time, the write data received by each of the secondary storage control devices will differ. Remote copy processing is asynchronous processing, performed with a timing different from the timing with which write requests are received from a primary host. Hence a plurality of remote copy tasks are each executed with arbitrary timing. As a result, circumstances arise in which write data which has been received by one secondary storage control device has not yet been received by another secondary storage control device.
Thus when simultaneously executing a plurality of remote copy processing tasks between a primary site and a secondary site, because the states of progress of the various remote copy processing tasks differ, it is difficult to maintain data consistency.